I found mouse droppings near our kitchen sink and in the cabinet below when I investigated further.
My guess? These are remnants from the same critter which apparently hitched a ride in the dog's bag (Yes, she has a bag we use for travel. It's a monogrammed zip-top LL Bean tote I used once to take food to a kennel where we were boarding her, and the kennel wrote her name on it in permanent marker. I'm not bitter much about that. We've since stopped taking her to that kennel, but for other reasons.) from the in-laws house at Christmas.
Thank goodness for bleach!
Of course it would be too much to ask for the cats to earn their weight around here by catching said mouse. Instead they seem to find their way to the empty fishtanks in the basement and die a slow painful death of dehydration and starvation. Funky.